This relates to commonly assigned U.S. Ser. No. 824,241 filed Jan. 30, 1986, now abandoned, and Ser. No. 943,794 filed Dec. 19, 1986, now U.S. Pat. No. 4,728,680, and the disclosure of said applications incorporated herein by reference. This invention pertains to pigmented emulsion coatings treated by cation exchange where the emulsion polymers contain hydroxyl or carboxyl groups but are free of amine groups. The stable reactive coating is based on ion-exchange of a water dispersed mixture of glycoluril resin and acrylic emulsion polymer, where the emulsion polymer contains carboxyl and/or hydroxyl groups but is free of amine groups, and the coating contains certain preferred titanium dioxide pigments.
Prior to this invention, glycoluril cured thermosetting emulsions were produced from non-ionic and acidic surfactants and generally required high levels of such surfactants. Anionic salt surfactants cause a detriment to the curing reaction. Further, pigmentation of such glycoluril and emulsion mixtures caused undesirable flocculation during the pigment grind step and subsequent let-down to produce pigmented mixtures.
Prior art U.S. Pat. Nos. such as 4,442,257 and 4,444,941 and 4,540,735 teach the use of certain acrylic latexes crosslinked with either tetramethylol glycoluril (TMGU) or dihydroxy dimethylol ethylene urea (DDEU) for low-temperature curing coatings. Rapid cure is achieved with these products by adding from 1.5% to 3.5% by weight of a 40% solution of paratoluene sulfonic acid where a package pH of about 1.0 to 2.0 results. However, stability of these catalyzed products ordinarily is limited to 1-5 days at room temperature, and much less at elevated temperatures. Hence, the acid catalyst must be added at the time of use. Cured films of such polymer mixture often exhibit certain water sensitivity due to residual catalyst in the cured films. Without the catalyst, however, the coating has no solvent or water resistance unless baked at extremely high temperatures.
Abbey (U.S. Pat. No. 4,525,260) discloses a cationic latex for cathodic electrocoating and specifically includes amine monomers. The reference latex is based on acrylic monomers copolymerized with amine monomers, such as N,N-dimethyl-2-aminoethyl methacrylate, and catalyzed with an azo catalyst. Abbey subsequently treats the cationic latex with an ion exchange resin but to specifically remove amino monomers and amino initiator fragments. Hence, the reference is specifically directed to removing amino fragments introduced in the emulsion process. As noted below, amine latexes are not operative in this invention in that amine groups block coreactivity with a glycoluril cross-linker and inhibit the cure.
In commonly assigned U.S. Ser. No. 824,241 filed Jan. 30, 1986 and U.S. Ser. No. 943,794 filed Dec. 19, 1986, low temperature cure emulsion polymers particularly useful in paint coatings are disclosed based on a thermosetting polymeric binder composition comprising a functional addition polymer containing functional hydroxyl or carboxyl groups, but free of amine groups, and a glycoluril derivative adapted to be coreacted with the functional polymer. The functional polymer and preferably the glycoluril derivative are treated with an ion exchange process step to remove undesirable cations. The resulting resin provides a highly desirable low temperature cure paint coating adapted to thermoset without the addition of undesirable acid catalysts. However, it has been found that cation exchanged polymeric mixtures based on functional emulsion polymers adapted to be coreactive with glycoluril derivatives and pigmented with considerable amounts of conventional commercial titanium dioxide exhibit an upward pH drift of as much as one pH unit within a few days after cation exchange of the polymeric binder and/or the paint coating, which does not seem to materially effect the stability of low heat cures, but does retard room temperature cure, and thus the commercial utility of process is effectively limited to darker color paints. In this regard, it has been found that conventional surface treatments of commercial titanium dioxide pigments include cations, particularly aluminum oxide cations, which eventually are attacked by acid and removed into the paint mixture thereby effectively increasing the pH of the paint mixture.
It now has been found that thermosetting pigmented emulsion dispersions useful as paint coatings containing glycoluril-type cross-linkers, where the mixture is subsequently subjected to cationic exchange to remove the cations from the anionic surfactant as well as from other sources can be substantially improved by incorporating opacifying pigments substantially free of cations on the pigment surfaces. The emulsion polymer specifically contains hydroxyl and/or carboxyl groups, but not amine groups, and is adapted to coreact with the glycoluril derivative. Amine groups block reactivity, inhibit the cure and render the process of this invention inoperative. In accordance with this invention, the ion-exchange process for removing undesirable cations from the polymeric binder is utilized in conjunction with specific opacified pigments substantially free of cations or cationic surface treatment. The cationic-free opacified pigments comprise untreated rutile and anatase titanium dioxide, or conventional surface treated commercial titanium dioxide rutile or anatase pigments with the cationic surface treatment removed to eliminate the cation rich pigment surfaces. By using untreated titanium dioxide or conventional titanium dioxide with cationic surface treatments removed, the pH drift is stabilized and commercially useful light color paints can be produced. Excellent room temperature cures can be achieved. Either rutile or anatase titanium dioxide, free of surface cations, particularly enables a substantially improved cure and cross-linking of the thermosetting paint coating. A further advantage of cation-free pigments is that the resulting pigmented paint coating is substantially free of thixotropic properties. These and other advantages of this invention are further illustrated by the detailed description of the invention and the illustrated examples.